The man who acted as a lookout and support person for the mastermind behind the murder of police accountant Curtis Cheng has been found guilty of conspiring with three others to plan and prepare a terrorist act.

Key points:

Mustafa Dirani had pleaded not guilty to acting as the "survellience man" in the October 2015 murder

Mr Cheng's widow Selina broke down after hearing the jury's decision

Mr Cheng was a 17-year veteran of the NSW police force's finance department

Mustafa Dirani had pleaded not guilty to planning and preparing a terrorist act and an alternative charge of supplying a firearm to an unauthorised person, but the jury delivered its verdict after almost three days of deliberations.

Dirani helped Raban Alou, the mastermind of the attack, and acted as a counter-surveillance man on October 2 — the day of the murder — while Alou attempted to buy a .38 Smith and Wesson revolver later used to kill Mr Cheng.

Mr Cheng's widow Selina Cheng broke down in the court after hearing the jury's decision.

Speaking outside court, Ms Cheng said although she and her family were relieved with the guilty verdict, "nothing will bring Curtis back".

"I would like to thank all the people who have worked so hard to give us this justice today," she said.

"Today is not a reason to celebrate at all, it is just an end to the court process. Nothing will bring Curtis back."

Mr Tedeschi argued that Alou and Dirani had arranged to have lunch and Dirani was just following him on the way to McDonalds.

Farhad, who fired several more shots at officers as they emerged from the building to respond to the incident, was shot dead by a special constable outside the police headquarters after he murdered Mr Cheng.

Earlier in the trial, the court heard that after the schoolboy was shot dead, police found a note inside the pocket of his robes that said he had come "to put terror into your hearts".

Mr Cheng was a 17-year veteran of the police force's finance department.

After the seven week trial, Justice Peter Johnson exempted all members of the jury from serving in court for the next eight years.